1. Field of the Invention
This invention relates generally to semiconductor fabrication and, more particularly, to a method and apparatus for loading or unloading a substrate holder that accommodates a plurality of semiconductor substrates.
2. Description of the Related Art
A substrate holder, such as a wafer boat or rack, is typically used to hold a plurality of semiconductor substrates, such as wafers, for processing in a conventional vertical heat treatment apparatus. The wafer boat commonly comprises a plurality of support accommodations that support the wafers at edge portions of those wafers. In this way, the wafers are held oriented horizontally in a vertically spaced-apart manner. In general, wafers are automatically loaded from a wafer container into a boat using a generic wafer handler, including an end effector for interfacing with the wafer, by contacting the wafer at its backside, or bottom surface.
Heat-related complications, however, can preclude the use of common wafer boats for very high temperature treatment or processing of a batch of wafers in a furnace, e.g., processing at temperatures up to about 1350° C. For example, due to the limited mechanical strength of the wafer at high temperatures, the wafer's own weight can cause it to plastically deform at very high temperatures because common wafer boats support wafers only at their edges.
Wafer boat wafer supports using other arrangements for supporting wafers are described in U.S. Pat. Nos. 5,865,321 and 5,820,367. U.S. Pat. No. 5,865,321 describes a wafer boat having a wafer support with multiple inwardly extending arms to support the wafer at more inward locations. U.S. Pat. No. 5,820,367 describes a wafer boat that supports a wafer at a central location using the entire circumference of a ring support. The supports in these wafer boats, however, are still not sufficiently supportive to prevent plastic deformation and consequent crystallographic slip of the wafer.
Wafer supports for single wafer systems can each support a substantial portion of the bottom surface area of a wafer and do not suffer from crystallographic slip to the same degree as wafers in conventional wafer boats. For such single wafer systems, susceptors supporting wafers over their entire bottom surface area and support rings forming a complete circle to support a wafer at its perimeter are known. Special measures, however, are required to place a wafer onto or to separate a wafer from such susceptors.
In particular, with such a single wafer system, when using a robot end effector, access to the wafer is generally provided from the bottom and the susceptor stays in place within a process chamber while wafers are serially loaded and unloaded. Typically, the robot end effector places a wafer on moveable pins at a level above the susceptor, the wafer being spaced sufficiently above the susceptor to allow enough clearance for retracting the robot end effector without the robot end effector touching the wafer or the susceptor. After retraction, the pins move downward to lower the wafer onto the susceptor. To unload the wafer, the reverse of these steps occurs. While suitable for single wafer processing, such a wafer loading and support system is not easily applicable to a batch processing system because, if possible at all, such a system would be unacceptably complicated and cumbersome, since it would require, inter alia, that every processing position be provided with moveable pins and the attendant hardware and control systems to move these pins.
Accordingly, there is a need for substrate holder systems and loading methods that, inter alia, provide improved support for substrates and that allow for efficient loading and unloading of the substrates for processing in a process chamber.